Sophisticated electronic information display devices are no longer the exclusive domain of engineers and scientists. The cathode ray tube and related optical display devices are now the almost universal accompaniment to the increasingly widespread data processing devices appearing in all areas of business and society.
Along with the relatively predictable economic impact of these display devices and their direct effects on the nature of data oriented work, there also has arisen some less well anticipated ergonomic dilemmas.
A particular example of this latter problem of human and machine interaction is the difficult matter of image contrast and resolution. For various reasons, the most common optical image display devices tend to produce a rather diffuse image. The devices are presently being used for long hours by a population with widely differing levels of visual acuity. They are also being used under various conditions of ambient light, including bright lights which produce extraneous images by reflection. Thus, the likelihood of visual fatigue sharpens the need for high contrast, high resolution screens with minimal distracting extraneous images.
Specifically, the visual sharpness of a typical cathode ray tube with a phosphor-backed screen is limited, among other things, by:
1. The inherently multi-directional nature of phosphor emission, combined with a refraction at the surface of the screen,
2. Halation or "ballooning" that results from off-angle light reflected from inside the screen,
3. External light reflecting from differentially refractive interfaces within the screen,
4. External light reflecting from the conductive metal coating, usually aluminum, on the inside of the screen, and
5. External light reaching the phosphor layer with the energy and frequency to excite a superfluous emission.
Prior display screen faceplates have addressed one or more of these; often with unacceptable trade-off disadvantages. Surface etching or frosting to reduce halation brings its own inherent diffusion to the image. Neutral density filters, while diminuating off angle rays, reduce the intensity of the highly desirable rectilinear, or normal, portion of the phosphor emission as well. A bandpass filter eliminates full spectrum color as a display option. The costly optical coatings which are used to reduce reflection and re-emission of external light are highly dependent for their effectiveness on the wave length and angle of entry of the external light. Light absorbing "louvres" or "venetian blinds" present an asymmetry in their effect with the angle of view, the image darkening more quickly with either a horizontal or a vertical change in the angle of viewing. Such devices, moreover, present costly manufacturing difficulties.
One of the most successful of the prior strategies for image enhancement in highly critical display applications has been the fiber-optic faceplate. Aside from the prohibitive cost for ordinary applications and the manufacturing complexity of this approach, a number of other problems have appeared. For example, since the optical fibers must be clad in a material having a different index of refraction from the core, there exist throughout the faceplate corresponding sharp gradients in the coefficient of thermal expansion, and a general difference from standard glass with regard to this property. Such screens are subjected, during manufacture and during use, to sometimes rapidly fluctuating, high amplitude variations in temperature. If such a screen consists of a faceplate laminated to an ordinary glass screen, a failure of the lamination can occur due to differential expansions and contraction. This may also lead to a failure of the integrity of the screen with regard to hard vacuum and resistance to high voltages common in such applications.
These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide an optical display screen which heightens the contrast and resolution of an optical display.
Another object of this invention is the provision of an optical display screen which acts as an oblique light filter for both internal and external light.
A further object of the present invention is the provision of an optical display screen which diminishes the inherent oblique components of phosphor emissions.
It is another object of the instant invention to provide an optical display screen which limits internal reflection and the attendant halation and does not increase surface diffusion.
A still further object of the invention is the provision of an optical display screen which has a highly symmetrical response to the angle of view.
It is a further object of the invention to provide an optical display screen which has no diminuating affect on near normal components of phosphor emission.
It is a still further object of the present invention to provide an optical display screen which allows full spectrum color display.
Another object of the invention is the provision of an optical display screen having dependable resistance to delamination and to vacuum and high voltage leakage.
Another object of the invention is the provision of an optical display screen which is simple and integral in construction, less expensive to manufacture than the alternatives, and which is capable of a long life of useful service with a minimum of maintenance.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.